1. Field of the Invention
The present invention relates to a rotary-cup coating apparatus for uniformly coating a coating solution on a surface of a planar workpiece such as a semiconductor wafer or a glass substrate.
2. Description of the Prior Art
Fabrication processes for forming devices on semiconductor wafers, glass substrates, or the like employ coating apparatus for coating or applying a coating solution on such planar workpieces.
One conventional coating apparatus includes a rotary spinner chuck disposed in a fixed cup for attracting a planar workpiece such as a semiconductor wafer on an upper surface thereof. While the planar workpiece is being attracted to the spinner chuck, a coating solution drops from a nozzle onto the upper surface of the planar workpiece, and the spinner chuck is rotated by a motor to spread the coating solution uniformly over the upper surface of the planar workpiece under centrifugal forces.
Since the cup is fixed in place and the spinner chuck rotates, turbulent air flows tend to be produced in the fixed cup. When such turbulent air flows are generated in the cup, they prevent the coating solution from being applied uniformly to the upper surface of the planar workpiece, and allow fine dust particles to be attached to the upper surface of the planar workpiece.
Japanese laid-open patent publication No. 3-293056 discloses a rotary-cup coating apparatus having a rotary cup assembly. The rotary cup assembly comprises a rotatable inner cup and a fixed outer cup disposed around the rotatable inner cup. A vacuum chuck for holding a workpiece thereon is mounted centrally in the rotatable inner cup for rotation therewith.
FIG. 3 of the accompanying drawings shows an improvement of the rotary-cup coating apparatus disclosed in the above publication. As shown in FIG. 3, the improved rotary-cup coating apparatus has a base 100, an inner cup 101 rotatably supported on the base 100, and an outer cup 102 fixed to the base 100 around the inner cup 101. The inner cup 101 has drain pipes 103 (one shown) projecting from an outer circumferential surface thereof radially outwardly into the outer cup 102. A chuck 105 which can be rotated by a motor 104 is disposed in a central opening defined in the bottom of the inner cup 101. The chuck 105 is vertically movable by a cylinder unit 106. When the chuck 105 is in an elevated position, it receives a semiconductor wafer. When the chuck 105 is in a lowered position as shown in FIG. 3, the chuck 105 engages the inner cup 101 through interengaging portions 107 for rotation therewith. In the lowered position, the chuck 105 and hence the inner cup 101 are rotated by the motor 104, and a coating solution applied to the upper surface of the semiconductor wafer is spread uniformly thereover under centrifugal forces.
In operation, the rotary force of the motor 104 is transmitted from the chuck 105 to the inner cup 101. The inner cup 101 has a larger diameter and a larger mass than the chuck 105. Therefore, when the inner cup 101 starts being rotated through the chuck 105 by the motor 104, the chuck 105 and the inner cup 101 cannot reach a desired rotational speed quickly. When the motor 104 is de-energized to stop rotating the chuck 105, since the rotational inertia of the inner cup 101 is very large, the interengaging portions 107 are subject to a large burden. Consequently, when the chuck 105 is repeatedly stopped, the interengaging portions 107 will eventually be broken.